@INPROCEEDINGS{2011:AMAM-BioBiped,
  author = {C. Maufroy and H.-M. Maus and K. Radkhah and D. Scholz and O. von Stryk and A. Seyfarth},
  title = {Dynamic leg function of the BioBiped humanoid robot},
  year = {2011},
  month = {Oct. 11-14},
  address = {Osaka, Japan},
  booktitle = {Proc. 5th Intl. Symposium on Adaptive Motion of Animals and Machines (AMAM)},
  abstract = {This contribution presents the concept and design of the first robot of the BioBiped series, aiming to transfer biomechanical insights regarding the mechanics and control of human walking and running to bipedal robot design and actuation. These are supported by preliminary experiments with the robot, where synchronous and alternate hopping motions could be successfully realized. This demonstrates that the robot design has the potential to develop dynamic gait patterns such as walking and running.},
}

@INPROCEEDINGS{2011:Humanoids-BioBiped1,
  author = {D. Scholz and S. Kurowski and K. Radkhah and O. von Stryk},
  title = {Bio-inspired motion control of the musculoskeletal BioBiped1 robot based on a learned inverse dynamics model},
  year = {2011},
  month = {Oct. 26-28},
  address = {Bled, Slovenia},
  booktitle = {Proc. 11th IEEE-RAS Intl. Conf. on Humanoid Robots},
  abstract = {Based on the central hypothesis that a humanoid robot with human-like walking and running performance requires a bio-inspired embodiment of the musculoskeletal functions of the human leg as well as of its control structure, a bio-inspired approach for joint position control of the BioBiped1 robot is presented in this paper. This approach combines feedforward and feedback control running at 1 kHz and 40 Hz, respectively. The feed-forward control is based on an inverse dynamics model which is learned using Gaussian process regression to account for the robot’s body dynamics and external influences. For evaluation the learned model is used to control the robot purely feed-forward as well as in combination with a slow feedback controller. Both approaches are compared to a basic feedback PD-controller with respect to their tracking ability in experiments. It is shown, that the combined approach yields good results and outperforms the basic feedback controller when applied to the same set-point trajectories for the leg joints.},
}

@INPROCEEDINGS{iros2011:radkhah,
  author = {K. Radkhah and O. von Stryk},
  title = {Actuation requirements for hopping and running of the musculoskeletal robot BioBiped1},
  year = {2011},
  pages = {4811-4818},
  booktitle = {IEEE/RSJ Int. Conf. on Intelligent Robots and Systems (IROS), Invited paper},
  abstract = {Actuation with variable elasticity is considered a key property for the realization of human-like bipedal locomotion. Also, an intelligent and self-stable mechanical system is indispensable. While much effort of current research has been devoted to the development of variable impedance joint actuators, this paper deals with the important question of how to determine the actuation requirements of a compliant, musculoskeletal robot that is targeted at fast dynamic motions. In a step-by-step approach, design decisions for the elastic humanoid robot BioBiped1 are presented. Using multibody system dynamics models and simulations, incorporating bidirectional series elastic actuator models and a realistic ground contact model, we analyze the actuation requirements of the employed electrical motors for computer generated hopping and human data based running motions. The numerical simulation results are accompanied by videos of the dynamics simulations. Recent experiments on the real hardware have indicated that the selected motor-gear units and elastic transmissions support the desired dynamic motion goals.},
}

@INPROCEEDINGS{2011:Lens-etal,
  author = {T. Lens and K. Radkhah and O. von Stryk},
  title = {Simulation of Dynamics and Realistic Contact Forces for Manipulators and Legged Robots with High Joint Elasticity},
  year = {2011},
  pages = {34-41},
  booktitle = {Proc. 15th International Conference on Advanced Robotics (ICAR)},
  abstract = {In this paper, multibody system dynamics simulation for manipulators and legged robots with high joint elasticities, particularly with focus on collision modeling, is addressed. We present the architecture of a newly developed toolbox in conjunction with a detailed discussion of a realistic contact, friction and stiction model, which is validated with real measurement data of a bouncing ball. The work presented is driven and inspired by two concrete robot developments in the authors" group: the manipulator BioRob and the biped BioBiped. The libraries are used to develop kinematic and kinetic models of these bio-inspired and highly elastic robots. Models and simulation of both robots are discussed, as well as occurring forces during collisions of the BioRob-X4 arm with the ground. We are also able to demonstrate good agreement of ground contact forces measured during slow jogging motion of a human subject with simulation results obtained with BioBiped1.},
}

@ARTICLE{2011:IJHR-BioBiped,
  author = {K. Radkhah and C. Maufroy and M. Maus and D. Scholz and A. Seyfarth and O. von Stryk},
  title = {Concept and design of the BioBiped1 robot for human-like walking and running},
  journal = {International Journal of Humanoid Robotics},
  year = {2011},
  volume = {8},
  number = {3},
  pages = {439-458},
  keywords = {Hopping; running; jogging; walking; biped; biomechanics; humanoid locomotion; compliance; mechanical elasticity; series elastic actuation},
  doi = {10.1142/S0219843611002587},
  url = {http://www.worldscinet.com/ijhr/08/0803/S0219843611002587.html},
  abstract = {Biomechanics research shows that the ability of the human locomotor system depends on the functionality of a highly compliant motor system that enables a variety of different motions (such as walking and running) and control paradigms (such as flexible combination of feedforward and feedback controls strategies) and reliance on stabilizing properties of compliant gaits. As a new approach of transferring this knowledge into a humanoid robot, the design and implementation of the first of a planned series of biologically inspired, compliant, and musculoskeletal robots is presented in this paper. Its three-segmented legs are actuated by compliant mono- and biarticular structures, which mimic the main nine human leg muscle groups, by applying series elastic actuation consisting of cables and springs in combination with electrical actuators. By means of this platform, we aim to transfer versatile human locomotion abilities, namely running and later on walking, into one humanoid robot design. First experimental results for passive rebound, as well as push-off with active knee and ankle joints, and synchronous and alternate hopping are described and discussed. BioBiped1 will serve for further evaluation of the validity of biomechanical concepts for humanoid locomotion.},
}

@TECHREPORT{2010:dd_tdp,
  author = {M. Friedmann and T. Hemker and S. Kohlbrecher and K. Petersen and S. Petters and K. Radkhah and M. Risler and D. Scholz and D. Thomas and O. von Stryk},
  title = {Darmstadt Dribblers - Team Description for Humanoid KidSize League of RoboCup 2010},
  year = {2010},
  institution = {Technische Universität Darmstadt},
  abstract = {This paper describes the hardware and software design of the kidsize humanoid robot systems of the Darmstadt Dribblers in 2010. The robots are used as a vehicle for research in control of locomotion and behavior of autonomous humanoid robots and robot teams with many degrees of freedom and many actuated joints. The Humanoid League of RoboCup provides an ideal testbed for such aspects of dynamics in motion and autonomous behavior as the problem of generating and maintaining statically or dynamically stable bipedal locomotion is predominant for all types of vision guided motions during a soccer game. A modular software architecture as well as further technologies have been developed for ecient and eective implementation and test of modules for sensing, planning, behavior, and actions of humanoid robots.},
}

@INPROCEEDINGS{rdkhh-kurowski:2010,
  author = {K. Radkhah and S. Kurowski and T. Lens and O. von Stryk},
  title = {Compliant Robot Actuation by Feedforward Controlled Emulated Spring Stiffness},
  year = {2010},
  volume = {6472},
  pages = {497-508},
  publisher = {Springer},
  booktitle = {Simulation, Modeling, and Programming for Autonomous Robots (SIMPAR 2010)},
  abstract = {Existing legged robots lack energy-inefficiency, performance and adaptivity when confronted with situations that animals cope with on a routine basis. Bridging the gap between artificial and natural systems requires not only better sensorimotor and learning capabilities but also a corresponding motion apparatus and intelligent actuators. Current actuators with online adaptable compliance pose high requirements on software control algorithms and sensor systems. We present a novel actuation mechanism and technique that allows for a virtual stiffness change of a deployed extended series elastic actuator without posing high energy requirements. The performance limits of the approach are assessed by comparing to an active and a passive compliant methodology. For this purpose we use a 2-degrees-of-freedom arm with and without periodic load representing a 2-segmented leg with and without ground contact. The simulation results indicate that the method is suited for the use in legged robots. },
}

@INPROCEEDINGS{2010:biorob_radkhah,
  author = {K. Radkhah and T. Lens and A. Seyfarth and O. von Stryk},
  title = {On the Influence of Elastic Actuation and Monoarticular Structures in Biologically Inspired Bipedal Robots},
  year = {2010},
  pages = {389-394},
  booktitle = {Proc. 2010 IEEE International Conference on Biomedical Robotics and Biomechatronics},
  abstract = {Implementing the intrinsically compliant and energy-efficient leg behavior found in humans for humanoid robots is a challenging task. Control complexity and energy requirements are two major obstacles for the design of legged robots. Past projects revealed that the control complexity can be drastically reduced by designing mechanically intelligent systems with self-stabilization structures. Breaking through the latter obstacle can be achieved by the development and use of compliant actuators. Mechanical elasticity and its online adaptation in legged systems are generally accepted as the technologies to achieve human-like mobility. However, elastic actuation does not necessarily result in energy-efficient systems. We show that mechanical elasticity, although being worthwhile, can have negative effects on the performance of drives. We present a methodology that introduces both elasticity and energy-efficiency to a bipedal model. To this end, we report on the influence of monoarticular structures and demonstrate that these structures have the potential to both take us a step further toward the goal of realizing human-like locomotion and reduce the energy consumption. },
}

@INPROCEEDINGS{Radkhah_Clawar:2010,
  author = {K. Radkhah and D. Scholz and A. Anjorin and M. Rath and O. von Stryk },
  title = {Simple yet effective technique for robust real-time instability detection for humanoid robots using minimal sensor input},
  year = {2010},
  pages = {680-689},
  month = {Aug. 31 - Sep. 03},
  address = {Nagoya, Japan},
  booktitle = {13th International Conference on Climbing and Walking Robots and the Support Technologies for Mobile Machines (CLAWAR2010)},
  abstract = {Legged locomotion of autonomous humanoid robots is advantageous but also challenging since it inherently suffers from high posture instability. External disturbances such as collisions with other objects or robots in the environment can cause a robot to fall. Many of the existing approaches for instability detection and falling prevention include a large number of sensors resulting in complex multi-sensor data fusion and are not decoupled from the walking motion planning. Such methods can not simply be integrated into an existing low-level controller for real-time motion generation and stabilization of a humanoid robot. A procedure that is both easily implementable using a minimal number of affordable sensors and capable of reliable detection of posture instabilities is missing to date. We propose a simple, yet reliable balance control technique consisting of a filtering module for the used data from two-axes-gyroscopes and -accelerometers located at the trunk, an instability classification algorithm, and a lunge step module. The modules are implemented on our humanoid robots which participate at the yearly RoboCup competitions in the humanoid kid-size league of soccer playing robots. Experimental results show that the approach is suited for real-time operation during walking. },
}

@INPROCEEDINGS{radkhah_ISR:2010,
  author = {K. Radkhah and M. Maus and D. Scholz and A. Seyfarth and O. von Stryk},
  title = {Towards Human-Like Bipedal Locomotion with Three-Segmented Elastic Legs},
  year = {2010},
  pages = {696-703},
  month = {Jun 7-9},
  address = {Munich, Germany},
  booktitle = {41st International Symposium on Robotics (ISR 2010)/ 6th German Conference on Robotics (ROBOTIK 2010)},
  abstract = {The long-term goal of the recently launched project BioBiped is to develop autonomous bipedal robots that are capable of energy-efficient multi-modal locomotion. In this paper we give a brief review of the important insights and techniques gained in previous and current projects leading to a new generation of human-like robots. Furthermore, we present the hardware design and the applied principles for the bipedal robot with three-segmented elastic legs that is currently under development. In the latter part of the paper we describe optimization methods that yield optimal parameter sets for tuning the walking and running gaits for a robot prototype with the same kinematic leg design. },
}

@ARTICLE{2010:IJMMS-Radkhah-etal,
  author = {K. Radkhah and T. Hemker and O. von Stryk},
  title = {Self-calibration for industrial robots with rotational joints},
  journal = {Int. J. Mechatronics and Manufacturing Systems},
  year = {2010},
  volume = {3},
  number = {3/4},
  pages = {187-209},
}

@INPROCEEDINGS{radkhah_Robio:2009,
  author = {K. Radkhah and S. Kurowski and O. von Stryk},
  title = {Design Considerations for a Biologically Inspired Compliant Four-Legged Robot },
  year = {2009},
  pages = {598-603},
  month = {Dec 19-23},
  note = {Finalist for Best Paper Award in Biomimetics},
  address = {Guilin, Guangxi, China},
  booktitle = {Proc. 2009 IEEE International Conference on Robotics and Biomimetics (ROBIO)},
  url = {http://ieeexplore.ieee.org/search/freesrchabstract.jsp?tp=&arnumber=5420606&queryText%3Drobio+Design+Considerations+for+a+Biologically+Inspired+Compliant+Four-Legged+Robot%26openedRefinements%3D*%26searchField%3DSearch+All},
  abstract = {In this paper we summarize some basic principles of legged locomotion in animals and then discuss the application of the principles to the design and fabrication of a four-legged robot. The here presented model combines ideas for better locomotion of robots both in the biologically inspired, mechanically intelligent structure and in the bionic controller. The movement of the legs is triggered by bionic drives with a setup similarly to biological muscles. The robot is characterized by several different gaits and an animal like locomotion without using feedback control. It has four legs, each having three joints of which two are actuated. During the development we also paid attention to the technical realization of the model. Special techniques to reduce the weight of the robot such as the achievement of different motions by changing the spring stiffness by means of intelligent control instead of an additional motor were also focused on during the development. Two novel features of our four-legged concept comprise the possibility of easily changing the spring stiffness deployed in the bionic drives of the joints and the way of this adjustment which requires neither complex computation nor additional motor. This feature allows the smooth transition to different gaits without necessarily having to change the controller parameters.},
}

@TECHREPORT{2009:dd_tdp,
  author = {M. Friedmann and K. Petersen and S. Petters and K. Radkhah and Dorian Scholz and D. Thomas and O. von Stryk},
  title = {Darmstadt Dribblers: Team Description for Humanoid KidSize League of RoboCup 2009},
  year = {2009},
  institution = {Technische Universität Darmstadt},
  abstract = {This paper describes the hardware and software design of the kidsize humanoid robot systems of the Darmstadt Dribblers in 2009. The robots are used as a vehicle for research in control of locomotion and behavior of autonomous humanoid robots and robot teams with any degrees of freedom and many actuated joints. The Humanoid League of RoboCup provides an ideal testbed for such aspects of dynamics in motion and autonomous behavior as the problem of generating and maintaining statically or dynamically stable bipedal locomotion is predominant for all types of vision guided motions during a soccer game. A modular software architecture as well as further technologies have been developed for efficient and effective implementation and test of modules for sensing, planning, behavior, and actions of humanoid robots. },
}

@INPROCEEDINGS{radkhah:2009,
  author = {K. Radkhah and T. Hemker and M. Friedmann and O. von Stryk},
  title = {Towards the deployment of industrial robots as measurement instruments - An extended forward kinematic model incorporating geometric and nongeometric effects },
  year = {2009},
  pages = {124-129},
  month = {Jul. 14 - Jul. 17},
  address = {Singapore, Singapore},
  booktitle = {Proc. 2009 IEEE/ASME Conference on Advanced Intelligent Mechatronics},
  url = {http://ieeexplore.ieee.org/Xplore/login.jsp?url=http%3A%2F%2Fieeexplore.ieee.org%2Fiel5%2F5211950%2F5229709%2F05230028.pdf%3Farnumber%3D5230028&authDecision=-203},
  abstract = {In the area of mounting and spot-welding of body-in-white, absolutely accurate robots are installed as measuring instruments, replacing expensive coordinate and other external measuring machines. Measurement technologies based on industrial robots play an increasingly important role. Such applications require highly accurate robots. Prior to deployment of highly accurate robot, however, it needs to be ensured that the implemented robot model fits the real model. Robot calibration can offer a significant opportunity to improve the positioning accuracy and to cut production costs. Existing calibration approaches fail to capture geometric and elastic effects occurring in the robot forward kinematics. Therefore, in this work an extended forward kinematic model incorporating both geometric and elastic effects has been developed in which the positioning accuracy of a manipulator, with or without an accurate internal robot model in the robot controller, is improved. },
}

@INPROCEEDINGS{radkhah:2008,
  author = {K. Radkhah and T. Hemker and O. von Stryk},
  title = {A Novel Self-Calibration Method for Industrial Robots Incorporating Geometric and Nongeometric Effects},
  year = {2008},
  pages = {864-869},
  month = {Aug. 05 - Aug. 08},
  address = {Takamatsu, Japan},
  booktitle = {Proc. IEEE International Conference on Mechatronics and Automation (ICMA)},
  url = {http://ieeexplore.ieee.org/stamp/stamp.jsp?arnumber=4798870&isnumber=4798708},
  abstract = {We propose a novel problem formulation for the solution of the nonlinear least squares appearing in the static calibration of the parameters of an industrial robot with rotational joints. A further contribution of the presented work is the use of an extended forward kinematic model incorporating both geometric and nongeometric parameters. These novelties facilitate the use of industrial robots as measurement systems. Both the automobile and the general industry are interested in the use of highly accurate robots that can be deployed as measurement instruments, e.g. for noticing incorrectly welded points early in the production process. Existing approaches are disadvantageous because of (1) the use of additional external measurement systems, and (2) the difficult, time-consuming, and cost-intensive determination of the robot base. Instead of absolute measurement data, we make use of relative measurement data by means of a camera attached to the robot flange. In tests based on data generated from simulation of a real experimental setup, we obtain absolute accuracies after calibration better than $\pm$ \unit[100]{$\mu$m}. This work lays the foundations for a cost-minimal and effective realization of a robot as a measurement instrument. },
}

@TECHREPORT{2008:dd_tdp,
  author = {M. Friedmann and K. Petersen and S. Petters and K. Radkhah and D. Thomas and O. von Stryk},
  title = {Darmstadt Dribblers: Team Description for Humanoid KidSize League of RoboCup 2008},
  year = {2008},
  institution = {Technische Universität Darmstadt},
  abstract = {This paper describes the hardware and software design of the kidsize humanoid robot systems of the Darmstadt Dribblers in 2008. The robots are used as a vehicle for research in control of locomotion and behavior of autonomous humanoid robots and robot teams with many degrees of freedom and many actuated joints. The Humanoid League of RoboCup provides an ideal testbed for such aspects of dynamics in motion and autonomous behavior as the problem of generating and maintaining statically or dynamically stable bipedal locomotion is predominant for all types of vision guided motions during a soccer game. A modular software architecture as well as further technologies have been developed for efficient and effective implementation and test of modules for sensing, planning, behavior, and actions of humanoid robots. },
}

@INPROCEEDINGS{2008-Kropf-etal-MMULATOR,
  author = {Matthias Kropff and Christian Reinl and Kim Listmann and Karen Petersen and Katayon Radkhah and Faisal Karim Shaikh and Arthur Herzog and Armin Strobel and Daniel Jacobi and Oskar von Stryk},
  title = {MM-ulator: Towards a Common Evaluation Platform for Mixed Mode Environments},
  year = {2008},
  number = {5325},
  pages = {41-52},
  month = {November},
  publisher = {Springer},
  editor = {S. Carpin et al.},
  series = {Lecture Notes in Artificial Intelligence},
  booktitle = {Simulation, Modeling, and Programming for Autonomous Robots (SIMPAR 2008)},
  url = {http://www.springerlink.com/content/62ph40794m636833/?p=72406ae46a8d4364bc570589871af29d&pi=7},
  abstract = {We investigate the interaction of mobile robots, relying on information provided by heterogeneous sensor nodes, to accomplish a mission. Cooperative, adaptive and responsive monitoring in Mixed-Mode Environments (MMEs) raises the need for multi-disciplinary research initiatives. To date, such research initiatives are limited since each discipline focusses on its domain specific simulation or testbed environment. Existing evaluation environments do not respect the interdependencies occurring in MMEs. As a consequence, holistic validation for development, debugging, and performance analysis requires an evaluation tool incorporating multi-disciplinary demands. In the context of MMEs, we discuss existing solutions and highlight the synergetic benefits of a common evaluation tool. Based on this analysis we present the concept of the MM-ulator: a novel architecture for an evaluation tool incorporating the necessary diversity for multi-agent hard-/software-in-the-loop simulation in a modular and scalable way.},
}

@INPROCEEDINGS{radkhah:2007,
  author = {K. Radkhah and D. Kulic and E. Croft },
  title = {Dynamic Parameter Identification for the CRS A460 Robot},
  year = {2007},
  pages = {3842 - 3847},
  month = {Oct. 29 - Nov. 2},
  address = {San Diego, CA},
  booktitle = {Proc. IEEE/RSJ International Conference on Intelligent Robots and Systems},
  url = {http://ieeexplore.ieee.org/Xplore/login.jsp?url=http%3A%2F%2Fieeexplore.ieee.org%2Fiel5%2F4398943%2F4398944%2F04399314.pdf%3Farnumber%3D4399314&authDecision=-203},
  abstract = {Dynamic Parameter Identification is a useful tool for developing and evaluating robot control strategies. However, a multi degree of freedom robot arm has many parameters, and the process of determining them is challenging. Much research has been done in this area and experimental methods have been applied on several robot arms. To our knowledge, there is currently no set of inertial parameters, either by modelling or by estimation, available for the CRS A460/A465 arm, a popular laboratory table top robot. In this paper we review and compare a number of methods for dynamic parameter identification and for generating trajectories suitable for estimating the identifiable dynamic parameters of a given robot. We then present a step by step process for dynamic parameter identification of a serial manipulator, and demonstrate this process by experimentally identifying the dynamic parameters of the CRS A460 robot.},
}